Method for manufacturing a cathodic arc coated workpiece

ABSTRACT

A method of manufacturing a coated workpiece utilizes a target made of an alloy which is substantially a one phase. Coating is achieved by cathodic arc evaporation of the target in an oxygen atmosphere.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 08/927,864filed Sep. 11, 1997 and now U.S. Pat. No. 6,602,390, which was acontinuation of application Ser. No. 08/493,668 filed Jun. 22, 1995, nowabandoned, which claimed priority from Swiss application number2'024/94-4 filed Jun. 24, 1994, which priority claim is repeated for thepresent application.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates:

to a process for coating out of a metal alloy target at least oneworkpiece with an oxide of a metal alloy;

to a process for the production of a target comprising a metal alloy,with the alloy in the target being present substantially in a singlephase;

to a preferred embodiment of the process for electrically conductingtargets in general;

to a coating apparatus for cathodic arc evaporation with a gas inlet,connected to an oxygen supply, into the vacuum recipient and with atleast one evaporation target comprising a metal alloy;

to a preferred embodiment of the apparatus with an electricallyconducting target in general;

to a process for operating a cathodic arc discharge; and

to a use of said operating process.

Definition: In the following, the term “phase” is to be understood tomean the “crystallographic phase”.

Oxides of metal alloys are conventionally deposited by reactivesputtering coating, electron beam evaporation, ion plating or by CVDprocesses as coatings. If attempts are made to deposit oxides of metalalloys with cathodic arc evaporation, numerous problems are encountered.

Attempts at controlling the movement of cathodic arc spots with theknown means, such as with magnetic fields, which are successful withcoating depositions of pure metal alloys or conducting nitrides are notsuccessful for oxides of metal alloys. The reason for this failure isthe known strong change of the secondary electron emission with thechange of the target surface oxidation leading to a hysteresis of thestate of the cathode surface.

In addition, the listed problems arising in alloy oxide coating are alsocharacterized by the arc burning stationarily at particular targetlocations causing increased droplet emission which leads tostoichiometrically uncontrolled and even metallic droplet deposits.

Great interest with respect to fabrication technology exists inprocesses for coating workpieces with layers of insulating alloy oxides,in particular of stoichiometric ones, since these exhibit greathardnesses, which is known, for example, from EP-A-0 513 662,corresponding to U.S. Pat. No. 5,310,607 by the same applicant.

According to these references, hard material coatings (“Hartstoff” inGerman) are suggested which are essentially formed by single or mixedcrystal oxides of an alloy, specifically by (Al, Cr)₂O₃.

It is known from the field of reactive cathodic sputtering technology tocontrol the poisoning of the metal target with non-conducting reactionproduct layers, in particular of interest here, with electricallyinsulating oxide layers, through reactive gas regulation. Such anapproach was found to be counterproductive in cathodic arc evaporation.Lowering the partial pressure of oxygen and the consequent processcontrol toward a metal mode during arc discharge evaporation increasesthe danger of localized burning and therewith the danger of dropletemissions and the jumping of the cathodic arc spot movement over verylarge distances on the target surface.

During reactive cathodic arc evaporation for the production of nitridecoating, working in an atmosphere of excess nitrogen is recommended.Transferring this concept to oxide coating of the type of primaryinterest here, namely primarily to alloy oxide coating, but alsogenerally to arc vapor depositions with insulating layers, such as, forexample, with non-conducting metal oxide layers, does not lead tosuccess since in the event of an oxide coating of the target or coatingwith non-conducting layers, the arc discharge frequently fails and,because of the poisoning insulation through the known ignition,mechanism, can no longer be reliably ignited.

These problems exist already in coating of workpieces with oxides ofpure metals by means of cathodic arc evaporation but are significantlymore pronounced if oxides of metal alloys are to be arc evaporated. Theintensifications of the problems with alloy evaporation compared tometal evaporation per se are also known from nitride coating technology.In this connection reference is made to O. Knotek, F. Löffler, H.-J.Scholl; Surf. & Coat. Techn. 45 (1991) 53.

It is known from JP 5 106 022 to evaporate a Ti—Al target by ion platingby means of a vacuum arc discharge and to deposit a TiAIN layer on ametal surface.

In “Cathodic arc evaporation in thin film technology” J. Vyskocil etal., J. Vac. Sci, Technol. A 10(4), July/August 1992, page 1740 acathodic arc evaporation is described.

In “Effects of target microstructure on aluminum alloy sputtered thinfilm properties”, R. S. Bailey, J. Vac. Sci. Technol. A 10(4),July/August 1992, page 1701 sputtered layers are addressed.

With respect to cathodic arc evaporation in which the cathode itself isvapor-coated, reference is made to EP-A-0 285 745, corresponding to U.S.Pat. No. 4,919,968.

From EP-A-0513 662, corresponding to U.S. Pat. No. 5,310,607 oxidecoating by means of crucible evaporation is known.

In principle, the use of cathodic arc evaporation for the production, inparticular, of metal oxide layers and, in particular, of layers of alloyoxides is extremely desirable, for one reason because the cathodic arcevaporation leads economically to high coating rates. In principle, alsoan improvement of the process stabilization of reactive arc evaporationcoating processes with insulating layers would be desirable.

SUMMARY OF THE INVENTION

It is the task of the present invention under all of its aspects, topermit coating workpieces, in particular with metal oxides and inparticular also with oxides of metal alloys, but also generally withinsulating layers from electrically conducting targets in astoichiometrically controlled way, and to implement this by utilizingthe advantages peculiar to cathodic arc evaporation, such as, forexample, their high coating rate.

For coating by means of an oxide of a metal alloy, this is attainedthrough a procedure according to the invention.

Surprisingly, it has been found that by using single-phase targets, incontrast to multi-phase targets, the cathodic arc spots move much moreregularly on the target which avoids burn-in and drastically reduces thedroplet density.

Although in some cases a limited quantity of other phases in the targetis not disturbing, their fraction, according to another feature of theinvention, should not exceed 30% or preferably 10%.

As will be explained in the following in conjunction with the examples,it was further found that generally the cathodic arc spot behaviorduring reactive arc evaporation of electrically conducting targets, inparticular of metal targets, and deposition of an electricallyinsulating reaction product in the form of a layer can be divided intotwo characteristic domains. Generally, it is possible to differentiateclearly between a domain with relatively low partial pressure of thereactive gas and a few cathodic arc spots, which jump over relativelylarge areas of the cathode or target surface, and a second domain ofrelatively high partial pressure of the reactive gas in which manycathodic arc spots move significantly faster and/or over smaller areason the cathode or target surface.

It has been found that the utilization of the second domain according tothe invention, practically completely prevents the formation ofdroplets.

According to the invention a multi-spot domain is optimally utilized,i.e. the process operating point is selected to be directly at thosepartial pressures of the reactive gas at which the arc discharge wouldfail.

Stabilization of the process operating point can be carried out throughobservation and control, but preferably through regulation, withobservation parameters preferably used or, in the case of a regulation,measured regulating variables as well as set variables during thecontrol (open loop), or manipulated variables set by means of regulationtechnology in the case of a closed loop. The present invention isfurther based on the task of suggesting a process for the production oftargets comprising a metal alloy with the alloy being present in thetarget essentially in a single phase.

According to another feature of the invention, the use of the abovestated process on aluminum/chromium alloys has been found to beexcellent.

According to a preferred embodiment a hard coating of said alloy with atleast 5 at % (atomic percent) chromium, preferably with 10 to 50 at %chromium, is deposited, the latter being excellently suitable, accordingto the above cited EP-A-0 513 662, viewed from the aspect of its layerproperties, for example for coating metal cutting tools.

The adhesion of said metal alloy oxide layer, in particular of the (Al,Cr)₂O₃ layer, as used on cemented carbide or ceramic bodies, such as areapplied for use of metal cutting tools, is significantly increased andbecomes more reproducible. Preferably a metallic intermediate layer of ametal/chromium alloy is deposited non-reactively but also with cathodicarc evaporation on the workpiece. Here too, preferably a target is usedon which the metal/chromium alloy, at least primarily, is present in asingle phase.

The layer succession is generated in the same coating chamber throughthe sequential contacting of the cathodic arc discharge onto thegenerally different targets and, for the deposition of the metal alloyoxide layer, the reactive gas oxygen is introduced into the treatmentatmosphere.

As has been stated, the deposition of non-conducting metal alloy oxidelayers within the scope of process stabilization is facilitatedsignificantly since the above described “multi-spot domain” is utilized.

But, according to another feature of the invention, this domain can beutilized generally for coating processes in which electricallyconducting targets are cathodic arc discharge evaporated in a reactivegas atmosphere and out of a reactive product a coating is deposited,which product is electrically non-conducting or at least is a poorerconductor than the evaporated target material.

A coating apparatus for cathodic arc evaporation with a gas inlet,connected to an oxygen supply, into the vacuum chamber and with at leastone evaporation target, in order to solve said task in terms ofinstallation technology, is distinguished according to another featureof the invention.

Preferred embodiments of this apparatus or installation are specifiedaccording to the invention.

As mentioned, on said installation a second target can be provided, inparticular with a metal/chromium alloy, preferably present primarily asa single phase, in order to deposit, apart from the metal alloy oxidelayer, an adhesion-enhancing intermediate layer on the workpiece.

With respect to another feature of the invention, the following shouldbe noted: because it was recognized according to the invention thatgenerally during coating with layers that are electrically poorerconducting than the target material, a reactive arc evaporation processis advantageously stabilized in said multi-spot domain, the invention isdirected toward an apparatus or installation in which generally anelectrically conducting target is provided, preferably utilized controlor regulating variables are specified with particular reference to theutilization of the discharge current frequency spectrum, as the measuredregulating variable or—in the case of a control—as the observedvariable, a variable which is significant for the characteristics ofoccurring cathodic arc spots and their movement.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained by example inconjunction with examples and figures, wherein:

FIG. 1 is a schematic illustration of an apparatus or installationaccording to the invention; and

FIG. 2 is a graph qualitatively plotting the arc voltage U_(B) and thepartial pressure of oxygen P₀₂ as a function of the mass flow of oxygenm^(o) ₀₂ or the axial magnetic field B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Apparatus and Configuration Used

In FIG. 1, a cylindrical vacuum coating chamber 1 can be evacuatedthrough a pump opening 23. Disposed therein are cathodes 2 and 3,fastened so as to be electrically insulated from the chamber, in theform of disks on a cover and a bottom of the apparatus, by means ofinsulators. They are each equipped with a cooling cavity 2′ and 3′ inorder to be able to carry off through a circulating coolant, theaccumulating heat loss. Both cathodes are connected to the negative poleof a current source 18 whose positive pole is conducted to annular disks4 (thus connected as anodes) encompassing the two cathodes, which carryoff the electrons from the gas discharge.

In addition, each cathode is advantageously equipped with a so-calledignition finger 15 (only the one for the upper cathode is depicted),which can be moved by means of an actuation device 16 carried throughthe chamber wall under a vacuum seal, in the direction of the arrow sothat the cathode can be touched with the ignition finger or it can beremoved from the cathode. The current flowing as a result is limited bya resistor 17 to a few 10 A. The interruption spark generated when theignition finger is lifted from the cathode subsequently becomes thefirst cathodic arc spot required for the evaporation.

The two cathodes 2 and 3 are each encompassed by a cylindrical metalsheet 19, mounted so as to be insulated from the chamber, and whichprevents the migration of the cathodic arc spots onto the cylindricalside wall of the cathode and in this way restrict the movement of themto the front face of the cathodes.

Furthermore coils 13 and 14 are present—they can be connected as aHelmholtz pair—which have the effect that already at low field strengthsof approximately 10 Gauss an increase of the plasma density and anincrease of the mutual coating rate of the two cathodes at constant arccurrent occurs.

Furthermore, in the coating chamber are rotatably disposed substrateholders 5 connected to a drive 6 in order to attain uniform coatingthrough rotational motion. On the substrate holders 5 are fastened theindividual holders 8 to 12.

2. Effects of Target Implementation

Through hot-forging, a target A having a diameter of 240 mm and athickness of 20 mm is produced from a powder mixture. The powder iscomposed of elementary Al and elementary Cr in a ratio of 55 percent byweight Al to 45 percent by weight Cr. After mechanical after treatment,the target surface was found to be permeated regularly with smallbreakouts on the order of magnitude of a few tenths mm. A small segmentof the target material produced in this way was separated. Its phasecomposition was determined by X-ray diffraction analysis. The spectrumcorresponds to a superposition of the spectrum of the cubicface-centered phase of aluminum and the cubic body-centered phase ofchromium.

Target B, having the same dimensions as target A, was also produced byhot forging, but this time from the powder of an alloy. The alloy iscomposed of 55 percent by weight Al and 45 percent by weight Cr and hadpreviously been produced by vacuum melting and ground in a protectivegas atmosphere to a grain size of a few tenths mm. Subsequently, thispowder was isostatically hot-pressed. A small segment of the targetmaterial produced in this way was separated. Its phase composition wasdetermined by X-ray diffraction analysis. The spectrum corresponds to amixture of γ-phases characteristic for Al—Cr alloys (see M. Hansen:Constitution of Binary Alloys, McGraw Hill 1958).

In an apparatus according to FIG. 1, however with only a single cathode,targets A and B were installed successively. A copper ring serves asanode 4 with slightly greater dimensions than the cathode and isdisposed concentrically with respect to it. The following dischargeconditions were chosen:

Arc current: 400 A Total pressure: 2 · 10⁻³ mbar argon.

With the aid of the magnetic coils 13 a magnetic field was generatedover the target-surface, which extends essentially radially outward, asdepicted in FIG. 1 at vector B.

The following was observed:

At target A, thus the two-phase target, the cathodic arc spots burnedlocally at intervals of a few seconds for approximately 1 second each,sometimes even significantly longer, at one site of the target surface.If the dwelling time of a cathodic arc spot lasted longer thanapproximately 5 seconds, the process was terminated manually in order toavoid strong local overheating of the target. By means of a reflexcamera the movement of the cathodic arc spots as a function of theshutter speed of the camera was recorded. At dwelling times of 1/15seconds and longer, five cathodic arc spots on average were observed.The mean speed of those cathodic arc spots which do not dwell at onesite, was only approximately 1 m/s.

After an operating time of approximately one hour, the bottom of theinstallation in region C according to FIG. 1 was strewn with solidifiedparticles consisting of target material. The maximum size of theseejected particles was approximately 2 mm. In addition, target A had avery porous surface. Subsequent scanning electron microscopy analysisstill demonstrated a two-phase surface.

At target B, burning of the cathodic arc spots for maximally a fewtenths seconds was rare, i.e. at most once every 5 minutes. The spotmovement was also significantly more uniform than was the case withtarget A and also significantly faster. The speed of the cathodic arcspots could not be determined at the available camera shutter speeds ofmaximally 1/60 seconds. The speed is presumably in the range of 10 to100 m/s. After an operating time of approximately one hour, no irregularsurface structure could be observed on target B, and in region C of theinstallation no droplets could be seen.

Conclusion:

Even without control of the arc evaporation process in a reactive gasatmosphere, it was found for evaporation of metal alloys that asingle-phase target yields significantly better cathodic arc spotbehavior than is the case with evaporation of a two-phase or amulti-phase target.

Consequently, in the following the investigations were carried out withsingle-phase alloy targets.

3. Effect of Process Control

The apparatus or installation according to FIG. 1, however with only onecathode, was equipped with a target according to B having a diameter of250 mm. The following operating conditions were set:

Arc current: 150 A Argon pressure: 0.18 · 10⁻³ mbar Magnetic fieldaccording approximately 40 Gauss to B of FIG. 1:

Dependence on Oxygen Flow

In FIG. 2 the function of the arc voltage U_(B) as a function of theoxygen mass flow m^(o) ₀₂ introduced into the installation according toFIG. 1 per unit time is shown qualitatively as is the dependence of thepartial pressure of oxygen P_(O2), the latter in a dash-dot line. Up toa critical flow f₁ the arc voltage U_(B) remains constant. Under theconditions selected, it was 38 V. With a further increase of flow m^(o)₀₂ (dm₀₂/dt) the arc voltage U_(B) increases continuously. At a secondcritical flow f₂ the discharge is extinguished, and the no-load voltageof the generator obtains, corresponding to U_(BO). This voltage is 60 Vin the present case.

Since the total pressure is essentially the sum of the unchanged argonpressure and the partial pressure of oxygen P_(O2), the latter remainsconstant up to the critical flow f₁, consequently also the partialpressure of oxygen P_(O2) stays constant. The partial pressure of oxygenP_(O2) is vanishingly small. Above the critical flow f₁ the partialpressure of oxygen also increases continuously and, in the present case,was 0.6·10⁻³ mbar at the critical flow f₂.

Observation of the arc discharge yielded a significant difference in theregion I below the critical flow f₁, and II, above said critical flowf₁: up to f₁ the discharge is characterized by few, i.e. two to five,rarely and relatively slowly jumping cathodic arc spots on the targetsurface. This behavior is typical for metal or nitride targets. Inregion II the discharge changes over into a fine network, becomingincreasingly finer, of large numbers, becoming increasingly larger, ofapproximately 40 to 100 cathodic arc spots which move much more rapidlyon the target surface.

Through the operation of the reactive arc discharge evaporation processin region II, and in particular in closest possible proximity to thecritical point corresponding to f₂, homogeneous droplet-freealuminum/chromium oxide coating is attained. But the operation of theprocess corresponding to point P of FIG. 2 in closest possible proximityto the critical point f₂ necessitates a regulated operating pointstabilization. If the requirements with respect to proximity of P to thecritical value f₂ are relaxed, in some cases a control of the processoperating point P can suffice.

Dependence on Magnetic Field

With the same configuration the effect of the magnetic field B wasinvestigated. This is basically an axial magnetic field whose magneticflux lines are perpendicular to the target surface. The arc voltageU_(B) increases with increasing magnetic field B so that in FIG. 2,instead of the mass flow of oxygen m^(o) ₀₂ on the X-axis with respectto the arc voltage U_(B), the strength of the magnetic field B can alsobe plotted.

Again, the qualitative characteristic, shown in FIG. 2, with respect toarc voltage at now constant mass flow of oxygen m^(o) ₂ results.

Therefrom the following possibilities can be derived for the processoperating point control or regulation:

a) The partial pressure of oxygen P_(O2) is determined as an observedvariable or, in a regulating loop, as a measured regulating variable andat least one of the following parameters is set within the scope ofcontrolling or regulating:

mass flow m^(o) ₀₂ of the oxygen,

arc voltage U_(B)

field strength B.

b) The arc voltage U_(B) is observed or recorded as a measuredregulating variable and at least one of the following parameters is setwithin the scope of controlling or regulating:

mass flow m^(o) ₀₂,

field strength of field B.

c) The frequency spectrum Sω of the arc current I_(B) according to FIG.1 is analyzed, for example the amplitude of a current spectral line atgiven frequency. Because the cathodic arc spot movement and, inparticular, its frequency and speed of jumping, is reflected in thefrequency spectrum of the discharge current, monitoring, for example,the amplitude of a frequency spectrum line in said current spectrumreveals the frequency with which the cathodic arc spots jump with thefrequency corresponding to said spectral line. In order to set thecathodic arc spot behavior so that cathodic arc spots jump with saidfrequency corresponding to the monitored frequency, in turn, at leastone of the following parameters is set:

arc voltage,

oxygen flow,

magnetic field strength.

As described, optimum process conditions are attained if the processoperating point P according to FIG. 2 is set to be in the closestpossible proximity to the break corresponding to the critical oxygenflow f₂.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A method for manufacturing a workpiece coatedwith an oxide of a metal alloy, comprising evaporating a target usingcathodic arc vaporization in an atmosphere containing oxygen, andproviding the target to be of an alloy material which is essentially ina single phase so as to reduce occurrence of droplets during reactivecathodic arc evaporation of the metal alloy material in the oxygencontaining atmosphere.
 2. A method as stated in claim 1, wherein thecathodic arc evaporating step uses an arc discharge which is operated inat least one of: a domain of a partial pressure of oxygen; a domainwhere a field strength of a magnetic field generated with a significantcomponent in the direction of the arc discharge; and in which at leastten cathodic arc spots are present on the target.
 3. A method as statedin claim 2, wherein the arc discharge is operated in at least one of thedomains so that at least nearly a maximum number of cathodic arc spotsis present on the target.
 4. A method as stated in claim 1, wherein a)the partial pressure of oxygen during the coating process is observedand deviation from a nominal partial pressure is minimized by setting atleast one of the following parameters: mass flow of oxygen, arc voltage,and field strength of a magnetic field substantially perpendicular tothe target surface; or b) the arc voltage is observed and deviation froma nominal conducting arc voltage is minimized by setting at least one ofthe following parameters: mass flow of oxygen, and said magnetic field;or c) a frequency spectrum of the discharge current is observed anddeviation of characteristic components of the spectrum from nominalcharacteristics is minimized by setting at least one of the followingparameters: arc voltage, mass flow of oxygen, and said magnetic field;and observing said variable and establishing said nominal value andforming said deviation and minimizing said deviation by said settingtaking place automatically by means of a process operating point closedloop.
 5. A method as stated in claim 1, wherein the alloy is present atleast as 70 at % in the one phase.
 6. A method according to claim 1,wherein the target is cast.
 7. A method according to claim 1, whereinthe target is formed thermomechanically from a pulverized intermetallicalloy compound.
 8. A method as stated in claim 1, wherein the alloy isan aluminum/chromium alloy.
 9. A method as stated in claim 1, whereinthe alloy comprises at least 5 at % chromium.
 10. A method as stated inclaim 1, wherein the alloy comprises at least 10 at % to 50 at %chromium.
 11. A method as stated in claim 1, wherein an oxide of analuminum/chromium alloy is deposited and that for increasing theadhesion of the deposited layer, a metallic intermediate layer isprovided as an adhesion layer.
 12. A method as stated in claim 1,wherein the workpiece is coated with a layer which is a poorerelectrical conductor than the evaporated target material which iselectrically conductive.